Colour Change
Colour change phenomenon in gemstones including alexandrite, colour-change garnet, sapphire, and diaspore with causes and grading.
Introduction
Colour-change gems display a different dominant hue depending on the spectral
composition of the illuminant, most famously shifting between daylight and
incandescent light. The benchmark example is alexandrite (colour-change
chrysoberyl), described as "emerald by day, ruby by night": it appears green
to teal under daylight or fluorescent light (6500 K) and red to purplish-red2700 K). [1]
under incandescent tungsten (
The mechanism requires a dual transmission window in the gem's absorption
spectrum: the stone transmits both red (650–700 nm) and blue-green450–510 nm) wavelengths simultaneously. Which colour the eye perceives
(
as dominant is determined by the spectral power distribution of the light
source in those two regions. In alexandrite this is caused by Cr³⁺ in the
chrysoberyl crystal field; in colour-change garnets (pyrope-spessartine
composition) it involves both Cr³⁺ and V³⁺. [2]
The degree of change, from weak (<50%) to complete (100%), is the primary
quality criterion. [3]
Mechanism
How colour change occurs:
The Physical Cause
- Gem's absorption spectrum has transmission windows in both red and green
- Daylight is balanced across the spectrum → green component dominates
- Incandescent light is red-rich → red component dominates
- Eye perceives the dominant transmitted colour [2]
Requirements
- Specific absorption spectrum with dual transmission windows
- Usually involves chromium, vanadium, or iron chromophores
- Balance between absorbed and transmitted wavelengths
- Light source spectral composition must differ significantly
Alexandrite
Alexandrite is colour-change chrysoberyl, the most famous and valuable
colour-change gem.
The Colour Change
- Daylight/fluorescent: Green to blue-green
- Incandescent: Red to purple-red
- Cause: Chromium (Cr³⁺) absorption
- Ideal: Complete change from pure green to pure red
Grading Colour Change
| Percentage | Quality Description |
|---|---|
| 100% | Complete change (pure green ↔ pure red) |
| 75-99% | Strong colour change |
| 50-74% | Moderate colour change |
| <50% | Weak colour change |
Quality Factors
- Degree of change: More complete = more valuable
- Attractiveness: Both colours should be appealing
- Saturation: Vivid colours in both lights
- Clarity: Eye-clean preferred
- Size: Large stones very rare
Alexandrite Sources
| Origin | Daylight Colour | Incandescent Colour | Notes |
|---|---|---|---|
| Russia (Urals) | Green | Red | Classic; historic; depleted |
| Brazil | Blue-green | Purple-red | Major current source |
| Sri Lanka | Olive/yellowish | Brownish-red | Often less distinct change |
| Tanzania | Green | Red | Some fine material |
| India | Variable | Variable | Emerging source |
Other Colour-Change Gems
| Gem | Daylight | Incandescent | Cause |
|---|---|---|---|
| Colour-change sapphire | Blue/violet | Purple/pink | Vanadium |
| Colour-change garnet | Blue-green | Red-purple | Vanadium + chromium |
| Colour-change spinel | Blue | Violet | Cobalt or iron |
| Colour-change diaspore | Yellow-green | Pink-red | Manganese |
| Colour-change fluorite | Blue | Purple | Rare earth elements |
Colour-Change Garnet
Some of the most dramatic colour changes occur in garnet:
Characteristics
- Usually pyrope-spessartine composition
- Can show blue-green to red-purple change
- Some match alexandrite's change quality [2]
- Relatively rare
Sources
- Madagascar (best known)
- Tanzania
- Sri Lanka
- USA (Idaho)
Testing Light Sources
Market Considerations
Colour-change gems in the market:
- Alexandrite: Extremely valuable; strong demand
- CC sapphire: Premium over standard sapphire
- CC garnet: Significant value when strong change
- Synthetic: Synthetic colour-change gems exist (alexandrite, sapphire)
- Identification: Chemical testing may be needed for species confirmation
References
- ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
- ↑ 2. Qiu, Z.; Guo, Y. (2021). Explaining Colour Change in Pyrope-Spessartine Garnets. Minerals, 11(8), 865. DOI: 10.3390/min11080865.
- ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.